Different methods and devices are used for characterizing adhesives. The most common test is the combined tensile and shear test. Two standard test strips are bonded together as test pieces and are pulled apart after hardening of the adhesive. The measured force up to the breaking point of the joint is a measure for the adhesive strength. This test is easily performed; however, due to the asymmetrical tensile load, twisting of the joint along the action of the force occurs in the two test strips. The tensile force no longer acts on the bonding surface at a right angle, which makes it impossible to draw an unambiguous conclusion about the adhesive's tensile properties.
A material which is mechanically stronger than the test strips is used in an approach known as the “thick combined tensile and shear test.” For forming a test piece, two oblong strips having a sufficient thickness and width are bonded on top of one another over the entire surface. Subsequent to bonding, both strips must be cut through perpendicularly to the longitudinal axis of the test piece in such a way that a defined friction-locked bonding joint is created between the cut-off points. When cutting, attention must be paid that the second test strip bonded to the particular test strip is not damaged.
Providing a test piece is thus highly complex and has the additional disadvantage that the accuracy in determining the effective bonding surface is dependent on the accuracy in creating the cut-off points in the individual test strips.
However, in very strong adhesive joints it may occur that the test strips have a lower mechanical strength than the bonding joint to be tested and break.
The availability of test pieces is thus highly dependent on the properties of the adhesive to be tested. In particular adhesives having high adhesive forces demand a correspondingly great preparation effort.
The plurality of tests for determining the desired properties of an adhesive to be performed in combinatorial chemistry is time-consuming, expensive, and associated with unsatisfactory test accuracy.